1. Field of the Invention
The instant invention relates to a modified silver catalyst useful in the epoxidation of ethylene to ethylene oxide and more particularly to a silver catalyst promoted with at least one higher alkali metal which shows a greatly improved selectivity in the process for producing ethylene oxide by direct oxidation of ethylene with molecular oxygen.
2. Description of the Prior Art
Supported silver catalysts have long been used for the air oxidation of ethylene and more recently in a so-called "oxygen process". Although the first reference to the use of silver as such a catalyst was made by Walter in British Pat. No. 21,941 (1905), it was not until some thirty years later that the first disclosures were made of the use of silver as a catalyst in the vapor phase oxidation of ethylene to ethylene oxide. See Societe Francaise De Lefort, U.S. Pat. No. 1,998,878 (1935).
A variety of techniques have been developed for the depositing of relatively small, but highly active amounts of silver on surfaces of non-silver support such as alumina. For example, McKim and Cambron in Canadian Journal of Research, Volume 27, Section B (1949) at 813-827, describe a method for depositing particulate silver on a support by decomposing silver oxalate in aqueous ethanolamine at 60.degree. C. and forming a paste which is applied to the surface of the support. In U.S. Pat. No. 3,043,854 issued July 10, 1962, to Endler, a silver coating formed by decomposition of a silver carbonate slurry is applied to a catalyst support surface.
It has been disclosed that supported silver catalysts can be prepared by impregnating a porous substrate with certain silver containing solutions and evaporating or decomposing the solutions to deposit silver on the substrate. U.S. Pat. No. 3,702,259 to Nielsen describes the use of an aqueous silver salt impregnating solution consisting essentially of a silver salt of carboxylic acid, an organic amine solubilizing/reducing agent such as ethylenediamine, a mixture of ethylenediamine or ethanolamine and ammonia or a mixture of ethylenediamine and ethanolamine. Van Bylandtlaan, in Belgium Pat. No. 808,278 (1974) employs an aqueous solution of hexamethylenetetramine with an ethylenediamine silver complex to deposit silver on an alumina support by decomposition. Additionally, it has been disclosed in Japanese Pat. No. 71/19,606 to Fujii et al that impregnation of inorganic supports with aqueous silver nitrate/alkanolamine complexes with subsequent thermal decomposition gives supported silver catalysts for ethylene epoxidation.
Recently it has been disclosed in British Pat. No. 1,413,251 to Nielsen and La Rochelle that certain alkali metals can be deposited on a refractory support coincidentally with the silver metal (U.S. Pat. No. 4,012,425).
Surprisingly, it has now been discovered that certain silver catalysts promoted with at least one higher alkali metal selected from the group consisting of cesium and rubidium are extremely stable, physically durable and highly selective in ethylene oxidation processes.
The promoted silver catalyst is easily prepared by impregnating a porous, inorganic substrate with a solution comprising:
(a) a silver compound,
(b) an organic amine solubilizing/reducing agent,
(c) a perchlorate salt of at least one higher alkali metal selected from the group consisting of cesium, rubidium and mixtures thereof sufficient to deposit on the said support an effective amount of the said higher alkali metal,
(d) an aqueous solvent and,
(e) a perhalogenated acid selected from the group consisting of perchloric acid, peroidic acid and perbromic acid in an amount of from 0 to 20 gram milliequivalent weights per milliequivalent weight of the perchlorate salt in the said solution. The impregnated support is heated to decompose the silver compound thus depositing silver on the substrate and activating the catalyst.
It has been found that the instant promoted catalysts provide outstanding selectivity in air oxidation processes or oxygen processes using diluent materials such as nitrogen, carbon dioxide, steam, argon, methane or ethane. This is particularly important in that such processes are not closed systems and some proportion of the unreacted ethylene is lost by venting excess gas. Additionally, the instant catalysts show high attrition resistance and surprisingly high mechanical strength.